It is well known to those skilled in the art that the most accurate way to install tension into a fastener in a joint is to control tightening by developing an indication of the tension in the fastener either directly from the fastener or another component used with the fastener. Fastener installation based on torque control can result in wide variations in the tension in the fastener because of variations in fastener lubricity, burrs on the thread of threaded fasteners, and variations in the friction at the surface of the joint component against which the bearing surface of a threaded fastener bears.
U.S. Pat. Nos. 3,187,621 and 5,015,132 are directed to pre-load indicating washers adapted for use with a threaded fastener and intended to develop an indication of proper tensioning of the fastener. These pre-load indicating washers each have a plurality of protuberances which are struck and partially sheared from the annular body to leave indentations in the annular body. With these pre-load indicating washers in a joint, the protuberances are pushed back into the indentations in the annular body from which they were formed as the joint is tightened and tightening is stopped when the protuberances have been "collapsed" back into the indentations in the annular body by a prescribed amount. This condition, typically, is sensed by a "feeler" gage inserted into a gap in the joint.
Under certain conditions, particularly with washers used with large-size fasteners, typically 1" in diameter and larger, these washers may produce inaccurate tension indications which fall outside the acceptable range of tensioning of the fastener. This is due to the protuberance not travelling back (i.e. "collapsing") uniformly into the indentation from which it was formed under the compressive load which is applied when the joint is being tightened. The protuberance, under compressive load, deforms plastically prematurely and partially collapses prior to the protuberance, as a whole, travelling back (i.e. "collapsing") into the associated indentation. The width of the protuberance/indentation combination of washers used with large-size fasteners is such that the mass of material at generally the center, which first feels the effect of the compressive load, cannot withstand the compressive load and begins to collapse prior to the protuberance, as a whole, travelling back into the indentation from which it was formed, leading to erratic and unreliable performance results.